Enhanced open circuit voltage by hydrophilic ionic liquids as buffer layer in conjugated polymer–nanoporous titania hybrid solar cells

다국어 초록 (Multilingual Abstract)

<P>We demonstrate the fabrication of a nanoporous titania (NP-TiO<SUB>2</SUB>) network structure by using a polystylene-<I>block</I>-poly(4-vinylpyridine) (PS-<I>b</I>-P4VP) diblock copolymer template and modifying the surface of NP-TiO<SUB>2</SUB> with ionic liquids (ILs), bmim-BF<SUB>4</SUB> and benmim-Cl. The effect of the molecular weight of PS-<I>b</I>-P4VP on the morphology of the NP-TiO<SUB>2</SUB> and IL-modified NP-TiO<SUB>2</SUB> are characterized by scanning electron microscopy and contact angle measurements. Subsequently, hybrid solar cells are fabricated using MEH-PPV and NP-TiO<SUB>2</SUB>, and the effect of IL layers and IL concentrations on device performances are evaluated under AM 1.5 G illumination. The devices containing bmim-BF<SUB>4</SUB> and benmim-Cl show drastically enhanced open circuit voltages (<I>V</I><SUB>oc</SUB>) of 1.05 V and 0.91 V, respectively, while the reference device without an IL layer exhibits a <I>V</I><SUB>oc</SUB> of 0.60 V. Significantly improved <I>V</I><SUB>oc</SUB> can be attributed to the change in interfacial energy levels by formation of ionic double layers at the TiO<SUB>2</SUB>/IL and at the IL/MEH-PPV interfaces. We also observed the trend that short circuit current decreased and <I>V</I><SUB>oc</SUB> increased with increasing IL concentration, which is ascribed to interruption of charge transfer from MEH-PPV to TiO<SUB>2</SUB> and the change in interfacial energy level by shifting the vacuum level, respectively.</P> <P>Graphic Abstract</P><P>The interfacial energy level of ionic double layers at the TiO<SUB>2</SUB>/MEH-PPV interface can change, resulting in remarkably improved open circuit voltages.
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